While thinking about what subsets of common languages, such as JavaScript and Java, could be considered pure, it occurred to me that object identity in most languages is an unexpected source of impurity. That is, if you're using object identity, you're not writing purely functional code.

We create two empty objects, and then determine whether they represent the same object using the triple equals operator (using is in Python or == in Java would have much the same effect). Since they were constructed separately, isTheSame holds the value false. Yet in a purely functional language, calling the same function with the same arguments (in this case, no arguments) twice should return the exact same value.

Strictly speaking, it's the construction of the object that is the impure code: calling the identity operator with the same arguments will always return the same value, whereas each time we construct an object, we assign it a different identity. However, code that either contains no use of object identity, or contains no construction of objects can be considered pure. Treating object identity as the impure concept that cannot be used is, in my opinion, the more useful option: it's quite handy being able to construct objects.